Bioleaching of complex zinc sulphides using mesophilic and thermophilic bacteria: comparative importance of pH and iron

This study investigates the bioleaching of the complex Pb/Zn ore/concentrate using mesophilic (at 30 °C), moderate (at 50 °C), and extreme thermophilic (at 70 °C) strains of acidophilic bacteria. The effects of bacterial strain, pH, iron precipitation, and external addition of Fe2+ on the extraction of zinc were evaluated. The results have shown that the ore is readily amenable to the selective extraction of zinc and lead using the acidophilic strains of bacteria [i.e., majority of lead (>98%) reports to the residue]. Moderate thermophiles displayed superior kinetics of dissolution of zinc compared with the other two groups of bacteria. The pH was found to exert a profound effect on the leaching process controlling the bacterial activity and precipitation of ferric iron mainly as K-jarosite. The K+ released presumably from the alteration of the silicate phases such as K-feldspar present in the ore appeared to promote the formation K-jarosite in moderately thermophilic leaching systems. The external addition of iron was shown to be required for the bacteria to efficiently drive the extraction of zinc from the bulk concentrate. These findings place the emphasis on the prime importance of ferric iron for the dissolution of zinc and of mineralogical properties (i.e., iron and silicate content) of an ore/concentrate to be treated via bioleaching processes.